1. Field of the Invention
The present invention relates to a laser scanning apparatus for use in image recording systems for recording an image on a sheet, image reading systems for reading an image on a sheet and the like, and more specifically to a laser scanning apparatus for causing a laser beam to scan a sheet in a direction perpendicular (the main scanning direction) to the direction of transport of the sheet (the subscanning direction) while the sheet is being transported, thereby recording an image on the sheet or reading an image on the sheet.
2. Description of the Prior Art
The following apparatus are generally known as such laser scanning apparatus.
Unexamined Japanese Patent Publication No. SHO 58-6660 discloses an apparatus comprising a drivingly rotatable subscanning drum and a rotatable nipping roller which are arranged with their axes disposed at a variable distance from each other. A sheet is fed to and positioned between the drum and the roller which are spaced apart at first, and the two members are thereafter pressed against each other to nip the sheet therebetween, whereupon the image on the sheet is scanned in the main scanning direction by a laser beam while the sheet is being transported in the subscanning direction by the rotation of the drum.
Another apparatus is known which comprises a subscanning drum and a nipping roller biased into pressing contact with the drum at all times by a spring or the like. A sheet is fed to the nip between the two members and is transported while being held therebetween.
Such image scanning apparatus must fulfill the requirement that the sheet transport speed be maintained at a constant value with extremely high accuracy, whereas the above-mentioned conventional apparatus are subject to the following problems.
In the laser scanning apparatus disclosed in the above-mentioned publication No. SHO 58-6660, the nipping roller which is at rest and spaced from the subscanning drum is brought into pressing contact with the rotating drum, with the sheet interposed therebetween, so that the resulting impact vibrates the drum, the sheet and the nipping roller. The nipping roller which is begins rotating further produces variations in the drum rotating torque. These phenomena vary the sheet transport speed which should be constant, producing irregularities in the density of scanning lines or in recording or reading spacing at the leading end portion of the sheet immediately after the start of scanning thereby causing disturbed images or images of impaired quality to be formed. Furthermore, the apparatus requires a complex mechanism, for example, for positioning the sheet.
The other of the foregoing apparatus wherein a spring or the like is used is subject to similar problems because the sheet forced into nipping engagement with the subscanning drum and the nipping roller vibrates the spring due to the elasticity thereof.